1. Field of the Invention
The present invention relates to color photography and more particularly, it relates to a novel class of two-equivalent magenta couplers which are suitable for use in silver halide color photographic light-sensitive materials.
2. Description of the Prior Art
It is known that, by the color development of a silver halide color photographic material, a color developing agent of an aromatic primary amine type is oxidized and is reacted with a coupler to from a dye, such as an indophenol, an indoaniline, an indamine, an azomethine, a phenoxazine, a phenazine and the like, thus forming a color image. In this type, the subtractive color process is ordinarily used for color reproduction and yellow, magenta and cyan color images are formed, which are respectively the complementary colors of blue, green and red. For example, a coupler of the acylacetanilide or dibenzoylmethane type is used for forming a yellow color image, a coupler of the pyrazolone, cyanoacetophenone or indazolone type is used for forming a magenta color image and a coupler of the phenol type, such as a phenol and a naphthol, is used for forming a cyan color image.
In one of the most preferred embodiments of the color photographic light-sensitive materials, the dye image forming couplers are incorporated into a silver halide emulsion. These couplers which are incorporated into the emulsion must be non-diffusible (diffusion resistant) in the binder matrix of the emulsion.
The color image forming couplers of the prior art are almost all four-equivalent couplers which require theoretically four moles of silver halide as an oxidizing agent for forming one mole of the dye through the coupling reaction. On the contrary a two-equilvalent coupler having an active methylene group which is substituted by a group capable of being released through the coupling of an oxidized product of an aromatic primary amino developing agent requires only the development of two moles of silver halide for forming one mole of the dye. Since the quantity of silver halide required for forming a dye with a two-equivalent coupler is one half of that required with an ordinary four-equivalent coupler many advantages are achieved with a two-equivalent coupler in that a thinner light-sensitive layer can be used and the layer can be processed rapidly. In addition, the photographic properties and economy can increased through a reduction in the layer thickness using a two-equivalent coupler.
Several attempts have been made to convert 5-pyrazolone type couplers, which have been conventionally used as a magenta color forming coupler, to two-equivalent couplers by substituting one of the hydrogen atoms at the 4-position of the 5-pyrazolone with a group capable of being split off. For example, in such couplers the 4-position of the pyrazolone is substituted with a thiocyano group as described in U.S. Pat. Nos. 3,214,437 and 3,253,924, an acyloxy group as described in U.S. Pat. No. 3,311,476, an aryloxy group as described in U.S. Pat. No. 3,419,381, a 2-triazolyl group as described in U.S. Pat. No. 3,617,291, a halogen atom as described in U.S. Pat. No. 3,522,052, and the like.
However, when these 4-position substituted pyrazolone couplers are employed, some disadvantages occur in that a remarkable color fog is produced, in that the coupling reactivity is insufficient, in that the couplers per se are chemically unstable and change into compounds which can not form dyes or in that many difficulties are encountered during the preparation thereof.
The substitution of the 4-position of the 5-pyrazolones with an alkylthio group, an arylthio group or a heterocyclic thio group is also known as described in U.S. Pat. No. 3,227,554. However most of these known thio-substituted pyrazolone compounds have the disadvantages that their reactivity with the oxidation products of an aromatic primary amine color developing agent is low, that the mercapto compounds which are formed upon the coupling reaction have such severe photographic effects that they can not be used in a conventional color photographic light-sensitive materials, and that the couplers are chemically unstable.
On the other hand, as couplers which can be used in a color developer solution, four-equivalent couplers have the disadvantage that a greater amount of silver halide is required to obtain a certain color image density, and the previously known two-equivalent couplers can not provide preferred results since the couplers are not sufficiently stable in a color developer solution.